The invention relates to a process for the preparation of copolymers of carbon monoxide with one or more compounds containing an ethylenically unsaturated bond.
The preparation of linear copolymers wherein units originating from carbon monoxide substantially alternate with units originating from ethylenically unsaturated compound(s) is known from various publications in the patent literature.
A convenient preparation method is described in EP 181.014. According to this method, a mixture of carbon monoxide and an alkenically unsaturated hydrocarbon is polymerized in the presence of a catalyst, containing one of the Group VIII metals such as palladium, cobalt or nickel; an anion of a carboxylic acid with a pKa lower than 2 and a bidentate ligand containing two phosphorus, arsenic or antimony atoms.
The copolymers prepared according to this method and similar preparation procedures, have relatively high molecular weights and exhibit attractive mechanical properties. They are suitable to be used as thermoplastics for many outlets, such as films, fibers and sheets, and shaped articles e.g. tubes, containers and car parts, and for applications in the domestic sphere.
For obtaining copolymers with optimal properties for the intended uses, it is generally preferred that variations in the average molecular weight of the copolymers produced are kept to a minimum. It would therefore be advantageous to perform the copolymerization under such conditions that the average molecular weight of the obtained product can be controlled during the process and substantially maintained at the desired value. The average molecular weight of the copolymers, calculated as number average (M.sub.n), typically is 10,000 or more.
The higher the molecular weight, the higher will be in general the intrinsic viscosity of the copolymers.
The intrinsic viscosity can be determined by dissolving the copolymers in m-cresol in different concentrations and measuring the viscosity of each solution at 60.degree. C. From the relative viscosities the inherent viscosities can be calculated and by extrapolating to a concentration of zero, the intrinsic viscosity in dL/g can be found. Instead of "intrinsic viscosity" the term limiting viscosity number (LVN) is used. The copolymers typically have an LVN of between 0.8 and 2.0 dn/g.
In addition to the aforesaid copolymers, oligomers of carbon monoxide and ethylenically unsaturated compounds may be prepared. Furthermore copolymers of these monomers having a considerably lower LVN than the above mentioned thermoplastic copolymers, e.g. in the range of from 0.2 to 0.6 dL/g, have properties which make them useful for other applications than the indicated outlets for thermoplastic products.
They may be used as such, or as starting materials for the preparation of plastics, as blending components or as plasticizers for other polymers. It would therefore also be advantageous, if the copolymerization process could be performed under such conditions that these products with lower average molecular weights, e.g. of 3000 or less, are the predominant reaction product.
In order to produce such lower average molecular weight copolymer, it has been proposed to increase the temperature at which the copolymerization is carried out. Although the formation of products with lower LVN's is thus enhanced, the stability of the catalyst system often becomes a problem.
It has further been proposed to reduce the molecular weight of the produced copolymers by addition of molecular hydrogen during the reaction. It appears that with most catalyst systems recommended for this process, a significant reduction in molecular weight only occurs by applying large amounts of hydrogen, say from 60 mol % or more, based on carbon monoxide. Apart from the unattractiveness of a high consumption of hydrogen, the use of large quantities of this gas often results in a reduction of the Group VIII metal component of the catalyst system, accompanied by a substantial inactivation thereof. Only in exceptional cases, by selecting specific ligands for the catalyst system, the intended reduction in molecular weight of the copolymers can be achieved, without impairing the catalytic activity of the system.
According to patent application (T-1862), i.e. U.S. Ser. No. 150,934 filed on Nov. 10, 1993, an adequate control of the average molecular weight of the formed copolymers is achieved and, if desired, products having relatively low molecular weights can be produced, by carrying out the copolymerization reaction in the presence of a minor amount of a compound containing a hydride moiety.